Finlayson was admitted to the Bar as a barrister and solicitor in February 1981. He was a partner in Brandon Brookfield from 1986 to 1990 and then in Bell Gully from 1991 to 2003. He has practised as a barrister sole at the Barristers.Comm chambers since 2003. At Bell Gully he spent years fighting for Ngāi Tahu against the government, pursuing its treaty claims through a series of high-profile court battles. "I used to love going to the office in the morning when we were suing the Crown" Finlayson said in a speech in 2009. "Ngāi Tahu mastered the art of aggressive litigation, whether it was suing the Waitangi Tribunal and [National Treaty negotiations minister] Doug Graham or the Director-General of Conservation. It was take no prisoners and it resulted in a good settlement."[4] The signing of the Treaty deal with Ngāi Tahu in 1997, he said in his maiden speech in parliament, was the highlight of his legal career.

Since his admission, Finlayson has appeared in all courts of New Zealand, including seven appearances before the Privy Council, including as counsel for the New Zealand Bar Association in Harley v McDonald [2001] 2 WLR 1749 and counsel for the British Government in Attorney-General for England and Wales v R (a decision of the Privy Council delivered on 17 March 2003). He has extensive experience appearing before tribunals and local authorities in New Zealand. He has had significant experience teaching in the Law faculty of Victoria University of Wellington. Since 1987, he has taught civil procedure for LLB, conflict of laws both for LLB and for LLM, intellectual property for LLB (Hons) and ethics for LLB. He served as Law Society representative on the New Zealand Council of Law Reporting from 1990 until 1998 and the New Zealand Council of Legal Education from 1992 until 1998.[citation needed]

Finlayson co-authored McGechan on Procedure, the leading New Zealand text on the practice and procedure of the Courts of New Zealand, and was the founding editor of the Procedure Reports of New Zealand. He has written papers on many subjects, including intellectual property, litigation and conflicts of interest and has presented New Zealand Law Society seminars on High Court practice, conflicts of interest and limitation.[citation needed]

Since his appointment as Attorney General, Finlayson has been successful in reaching an unprecedented number of financial Waitangi Treaty settlements with many Maori iwi he had represented in private practice. He has also used his executive powers to make more High Court applications seeking litigants be prevented court access on grounds they are vexatious than in the prior 60 years,[5][better source needed] including a protracted court battle against legal news blogger Vince Siemer.

Finlayson joined the National Party in 1974 while still at St Patrick's College, after having had a long conversation with Keith Holyoake at Parliament the previous year.[3] Finlayson stood as National's candidate for the Mana electorate in the 2005 election, and was also ranked twenty-seventh on National's party list, making him the second most highly ranked National candidate who was not already an MP. While he failed to win Mana, losing by a margin of 6,734 votes,[8] the National Party polled well on party votes and Finlayson was elected via the party list. Upon his election, he was appointed shadow Attorney-General by then leader Don Brash. He had previously held the Shadow Cabinet roles of Shadow Attorney-General, Shadow Treaty Negotiations Minister and Shadow Arts, Culture and Heritage Minister. And he was the Deputy Chairman of the Justice and Electoral Select Committee.

Initially ranked 27th on the party list, Finlayson was promoted to 18th in the first caucus ranking after the 2005 general election. Following the election of John Key as leader, Finlayson was promoted to the Shadow Cabinet and placed at 14th on the 2008 election party list.

Finlayson has described himself as being an "odd fish" since he is gay as well as being a Catholic.[3] He has no partner, and says he is celibate.[3] He is a distant cousin on his mother's side of Labour MP Annette King, with whom he traded insults in 2013.[13]